1. Field of the Invention
This invention relates to a side pincushion correcting circuit for color television receivers, and more particularly relates to an improved side pincushion correcting circuit for beam index type color television receivers. This invention is especially directed to such a correcting circuit which can correct side pincushion distortion almost completely while simultaneously correcting any variation in horizontal amplitude.
2. Description of the Prior Art
A beam index type color television receiver uses a picture tube havng a phosphor screen on which there are disposed red, green, and blue vertical color phosphor stripes which are scanned in the horizontal direction, as well as index phosphor stripes disposed on the inner surface thereof which are also scanned in the horizontal direction. The picture tube has a single electron beam which scans at least one such index phosphor stripe to produce an index signal. This index signal is supplied to, for example, a phase-locked loop to derive therefrom an output signal which is adapted to change-over the video color signal applied to modulate the beam, so that when the red color phosphor stripe is scanned by the electron beam, the red primary color signal is supplied to the picture tube, when the green color phosphor stripe is scanned, the green primary color signal is supplied to the picture tube, and when the blue color phosphor stripe is scanned, the blue primary color signal is supplied to the picture tube, for density modulation.
The frequency of the index signal is in inverse proportion to the pitch of index phosphor stripes and in proportion to the scanning speed of electron beams. Therefore, when side or horizontal pincushion distortion causes the scanning speed of the electron beam to change at each line, that is, as the position of the beam changes in the vertical direction, the frequency of the index signal is changed at each line (in the vertical direction) and in extreme instances exceeds the lock-in range of the phase-locked loop so that accurate color reproduction is likely to be impaired. Particularly, when the picture screen is dark and the index signal is weak, the lock-in range of the phase-locked loop is reduced and hence the above likelihood of impaired color reproduction is increased.
In the prior art, a conventional television receiver is adapted to correct side pincushion distortion in such a manner that a supply voltage furnished to the horizontal deflection circuit is modulated by a parabolic waveform having a (or field) period, and a saturable reactor is connected in series to a horizontal deflection coil so that the latter affects the horizontal deflection of the electron beam in accordance with the parabolic waveform with vertical period. However, satisfactory correction of side pincushion distortion in a beam index type picture tube can not be achieved by these methods. Additionally, changes in horizontal amplitude due to changes in high voltage or the like must be separately corrected.